The International Spaceflight Museum in Second Life was one of the first places to really ignite my passion for virtual worlds, as places where real education and public engagement can occur. A gigantic rocket park, your avatar can fly around getting an in-depth and otherwise impossible view of dozens of current and historic spacecraft. Basically it's a virtual Disneyland for space nerds.
Sadly, the Spaceflight Museum vanished from SL sometime in January because they didn't have the funds to continue to pay the virtual land costs that Linden Lab charges. But good news! On the ISM official blog, they recently reported that they are going to be able to re-open their sims in SL very soon because of a very generous anonymous donor. However they aren't out of the woods yet.
If you would like to support this historic and still very relevant museum in Second Life, consider going to their website to donate, contributing via PayPal to firstname.lastname@example.org, or by “paying” L$ in SecondLife to AyeEss Emms, the avatar that is used to manage ISM’s land owning account. I'm going to do just that myself.
UPDATE Feb 27 2012: According to New World Notes, the ISM has been saved from the wrecking block by an anonymous donor!